In the Bahá'í Faith, the greatest, darkest evil is that of covenant breaking. This most evil of acts, which may be perpretrated by Bahá'ís and Non-Bahá'ís alike, is defined best as a severe attack on the leadership of the Bahá'í community.
The remarkable characteristic of this great evil of evils is its contagiousness. Whereas the Christian Satan may be crafty and cunning, the Bahá'í Satan will get you by simply breathing into your ear. This can be a frightening prospect to Bahá'ís. It is quite effective in deterring them from communicating from people who have been branded covenant-breakers, but introduces a high price to be paid in fear.
Bahá'u'lláh and the Master in many places and very emphatically have told us to shun entirely all Covenant-breakers as they are afflicted with what we might try and define as a contagious spiritual disease ...
From a letter dated 30 November 1944 written on behalf of the Guardian to an individual believer
And in more detail:
Thou hadst asked some questions; that why the blessed and spiritual souls who are firm and steadfast, shun the company of degenerate persons. This is because, that just as the bodily diseases, like consumption and cancer are contagious, likewise the spiritual diseases are also infectious. If a consumptive should associate with a thousand safe and healthy persons, the safety and health of these thousand persons would not affect the consumptive and would not cure him of his consumption. But when this consumptive associates with those thousand souls, in a short time the disease of consumption will infect a number of those healthy persons. This is a clear and self-evident question.
Likewise, if a thousand magnanimous persons associate with a degraded one, the perfection of those souls will not affect this debased person. On the contrary, this mean person will be the cause of their going astray. Therefore, His Holiness Bahá'u'lláh says in the Tablets: "Soon will a foul odour be spread; shun it, so commandeth the Omniscient and the Wise. That is, in that city a stinking odour will soon be spread. You should avoid it. So are ye commanded by His Holiness, the Knower and the Wise." That foul odour is that of violation. Also in the Tablet of Advice He says: "Now, do not neglect your Sower, Protector, and Educator; and do not choose and prefer others to Him, lest foul and poisonous winds should pass over you."
His Holiness Christ says that the owner of the garden does not leave the dried tree, but certainly cuts it and throws it into the fire, because the dried wood is worthy and deserving of fire.
Again, His Holiness Bahá'u'lláh says: "Then, O ye trees of the blessed garden of My bestowal, protect ye yourselves from the poison of the treacherous souls and the stinking winds, which are the association of the polytheists and the negligent ones, so that the trees of existence, through the bounty of the Worshipped [God] be not deprived of the blessed breaths and breezes of love."
`Abdu'l-Bahá, tablet quoted in Star of the West , vol. XII.